One of the biggest problems of vehicles using fossil fuels, such as gasoline and diesel, is creation of air pollution. In order to solve this problem, a technology of using a re-chargeable secondary battery as a power source of vehicles has attracted considerable attention. Accordingly, an electric vehicle (EV), which is driven only by a battery, and a hybrid electric vehicle (HEV), which combines a conventional engine with a battery, have been developed and some thereof are now being commercially used. A nickel-metal hydride (Ni-MH) secondary battery has been mainly used as the power source of EV and HEV. Recently, use of a lithium-ion battery has also been attempted.
High output and large capacity are needed for such a secondary battery to be used as the power source of EV and HEV. To this end, a battery pack having a structure, in which a plurality of small-sized secondary batteries (unit batteries or battery cells) are connected in series and/or in parallel, is used.
The battery pack may include a plurality of battery cells to which leads are connected, and the leads of the battery cells may be welded to each other.